Wheelchairs are generally provided with a pair of non-swivelling wheels, such as drive wheels in the case of powered wheelchairs. Wheelchairs may also be provided with caster wheels, in particular swivel caster wheels facilitating rotation of the wheelchair in any direction.
Caster wheels of the type that swivel are known to flutter or to shimmy. Flutter or shimmy occurs when a caster wheel starts to oscillate side-to-side. In the case of flutter, the side-to-side oscillation is obtained by forced oscillation and in case of shimmy by self-excited oscillation. These side-to-side oscillations are highly undesirable because the direction of orientation of the caster wheel becomes uncontrollable. Further, the oscillations may give rise to vibrations in the seat which may be experienced as uncomfortable for the user. Normally oscillation occurs when the wheel is not in full contact with the ground, and the wheel may have turned in any direction when full ground contact is regained. Oscillations may also occur without the caster wheels losing contact with ground in case the wheelchair speed is sufficiently high.
There are a number of methods of reducing flutter and shimmy. U.S. Pat. No. 6,944,910, for example, discloses a caster wheel assembly for a wheelchair, including a housing, a caster fork configured to mount a caster wheel for rotation, and a caster bolt connected to the caster fork and inserted into the housing for rotation within the housing. The assembly also includes a damping insert applying a biasing force between the caster bolt and the housing to dampen rotation of the caster bolt with respect to the housing. The damping insert comprises a plurality of generally stiff but somewhat flexible discs. The discs are substantially flat, with major faces oriented substantially vertically. The discs are sufficiently stiff so that they resist bending. When the discs are inserted into the annular space between the caster bolt and the housing, their horizontal dimension exceeds the maximum chord length within the annular space, and the discs must be bent in order to fit into the space. This bending creates a biasing force between the rotatable spacer and the non/rotating housing, and the biasing force dampens the rotation of the spacer and the bolt with respect to the housing. The damping insert hence reduces caster wheel oscillations by means of friction.